What is the theme in The Thirteenth Tale by Diane Setterfield?

Twins are an important theme in this book. This novel is the story of two women, both of whom claim to be twins for a large portion of the novel. The first is Margaret Lea, the only child of a bookshop owner and his severely depressed wife. Margaret learns when she is still a child that she once had a twin who died shortly after birth. Margaret's parents never told her about this twin, but with the knowledge she realizes she has always known that a part of her was missing. Later in life, Margaret learns that she and her twin were conjoined and that during the separation proceedure her sister was killed. Margaret feels a great deal of guilt for the death of her sister, but she also feels as though a part of her is missing, that she is not whole.